One useful indicator of a person's health is percentage of body fat. One technique for measuring a person's percent body fat is the so-called “bioelectrical impedance” technique. According to this technique, a person's body fat is measured by determining the impedance of the person's body in response to electrical signals. The percent body fat is calculated according to a given formula based upon the measured impedance and other variables such as height, weight, age and sex, for example. Body impedance is typically determined by supplying a constant current through at least two electrodes that contact the body, thereby causing a voltage to develop across the body. This voltage is measured either (1) via the same electrodes through which current is supplied, or (2) via one or more pairs of voltage-measuring electrodes. The body impedance is then calculated from the current and the measured voltage, and the percent body fat may, in-turn, be calculated from the body impedance.
A two wire system has one pair of electrodes that are used for both excitation and sensing. A four wire system has separate pairs of electrodes for excitation and sensing, essentially decoupling the excitation from the response. In the case of a four wire system, current is driven into the body through a pair of excitation electrodes, and sensed independently through a second, independent pair of electrodes. A four wire system typically uses sinusoidal drive waveforms which have no average value, or DC offset, and operates in a sinusoidal steady state such that the resulting potential measured is a sinusoid, as is the magnitude and phase angle relative to the excitation.
By way of further, non-limiting example, at a frequency of 50 Kilohertz (50 Khz), the body impedance is approximately real, since the phase angle is close to zero degrees and the reactive component is extremely small and essentially negligible. The utilization of a four wire system also negates the effects of contact potential changes at the body (e.g., skin)/electrode interface. The electrode is actually operable as a sensor, as it converts electron current in the wire to ionic current in the body. At the interface, a standard cell potential is developed similar to what one would find in an electrochemical cell, such as a battery. The body impedance at 50 KHz, less than 1 milliamp (1 mA) current, is typically less than 1000 ohms (Ω) with a small reactive term. Essentially the impedance under these conditions is real with a negligible reactive term, and hence has a small phase angle (arctangent IM/RE where IM is the Imaginary part and RE is the Real part of the signal component values).
In contrast, conventional two wire systems operate as transient systems having a pulse-recovery response. In such a configuration, a capacitor charges through the body. The current level is low as the body under these conditions represents higher impedance. In some cases the body is not purely resistive and instead can be represented as a series resistor/capacitor network. Typically, the resistive component of the body at these levels and under pulse transient conditions is on the order of hundreds of thousands of ohms (100's of kΩ), and the model is not purely resistive. There is a charge and discharge recovery transient and associated system time constant. The net-effect from a circuit standpoint is a time constant with an impedance of about two orders of magnitude higher than a corresponding four wire system.
Repeatability of measurements in two wire systems can prove problematic. For example, people having thick skin at the body/electrode interface (e.g., calloused feet) may have their impedance measurements skewed since the electrode-skin interface and associated contact potential is not cancelled, as it is in a four wire measurement system. The resistance variations of the electrodes are irrelevant in a four wire Kelvin Bridge measurement system. However, in a two wire measurement system, such variations have a tangible effect, both in capacitance and resistance.
The Body Fat Equation (BFE) for a four wire system has a different weighted coefficient for the impedance term than that for a two wire system, since the impedance measurements have values that are greatly lower than in the two wire case. The sensitivity and span of the impedance measurement for the four wire approach is much less than that of a two-wire system as well.
Accordingly, an alternative two wire approach and circuit for measuring body impedance utilizing a two wire system is believed to be desired.